1. Field of the Invention
This invention relates to an improved mosaic or matrix print head. Such a print head generally includes a needle guide assembly and a plurality of electromagnetic structures coupled to the guide assembly. Each electromagnetic structure is mounted on an electromagnetic support and comprises a magnetic circuit formed by two pole pieces connected by a yoke and by a movable armature which completes the magnetic circuit. It further includes at least a coil wound around one of the two pole pieces.
2. Description of the Prior Art
Examples of mosaic or matrix print heads are given in U.S. Pat. No. 4,051,941, British Pat. No. 1,447,661 and U.S. patent application Ser. No. 74,264 filed by the same applicant. It should be noted that each of the electromagnetic structures acts as an activator for one of the printing needles. Furthermore, each movable armature is provided with at least an arm which protrudes externally to the electromagnetic structure and operates as an actuating arm for the associated needle. The needles are generally mounted on a carriage which is moveable along the printing line, in order to execute the serial printing of characters.
Mosaic print heads must be very inexpensive and simple in construction. At the same time they must allow for precise adjustment, in particular of the air gap width in the electromagnetic structure with the armature in the rest position and therefore the stroke of the associated printing needles. Such needle stroke adjustment is essential in order to achieve a very high operation frequency because the maximum attainable frequency is greatly influenced and limited by such stroke. In the previously identified U.S. patent application, a mosaic printing head is described where the adjustment of the electromagnetic gaps in the rest position is performed in a collective manner. In such case, however, it is not possible to adjust for the differences of the working tolerances of both needles and electromagnetic structures. As a result, this leads to stroke values very different for the several printing needles which create a defective type formation that becomes even more objectionable as the printing speed is increased. In fact, in a very fast matrix printing head if each needle has a different stroke, the transversal printing line corresponding to the seven needle line will not be perfectly aligned because the impact of the different needles against the printing support will not be simultaneous owing to the different strokes of the printing needles. Therefore, for very high printing speeds, it is essential that there be provided individual rest air gap adjustments for each electromagnetic structure and its respective needle. A solution of this kind is described in the English Pat. No. 1,477,661 but the head resulting from it is complex and expensive, and it requires a long time for the set up of each printing needle stroke.
Italian Pat. No. 956,585 describes a wire matrix print head where such adjustments can be individually performed on each electromagnetic structure and related needle. However, such print head is complex and expensive. In contrast, the number of constructive elements and constructive complexity are reduced to a minimum in the print head disclosed in U.S. Pat. No. 4,051,941 at the expense of adjustability and calibration.
Another solution of such kind is described in the French Patent Publication No. 7440815, wherein a reduced number of components and constructive simplicity are stressed. However, this design approach is not useful because it can not prevent the adverse vibration effects on the air gap adjustments.